bug#50880: Add "Contributing to Emacs" help command (see C-h C-h)

[Gregory Heytings]

> What was wrong with
>
> C-h g -> About the GNU project
> C-h G -> Emacs copying permission (GNU General Public License)
> C-h C-c -> Contributing
>
> ?

Another option:

C-h g -> About the GNU project
C-h C-l (as in "license") -> Emacs copying permission (GNU General Public 
License)
C-h C-c -> Contributing to Emacs